Every six months or so, there's a new spate of rumors about Microsoft bringing a full version of Office to the iPad.

The latest round kicked off last week, when some ambiguous comments by Microsoft marketing boss Tami Reller seemed to suggest Microsoft was going to save a full touch-screen version of Office for Windows first. Not so, said ace Microsoft reporters Mary Jo Foley and Paul Thurrott, both of whom reported that Microsoft has accelerated development on a touch-enabled version of Office for iPad, and will release it in the first half of this year -- before the next update to Windows comes out.

Too bad that this is a product with no market.

It's fun to get excited about Microsoft creating a whole new version of one of its most important products for a rival platform, but apart from making for an interesting business case study and debate topic for hardcore Microsoft and Apple followers, what useful purpose will a touch-enabled version of Office on the iPad serve?

The iPad was built for completely different usage scenarios than Microsoft Office. Its strengths are its portability and ease of use, as well as a huge selection of simple single-function apps. It's used for quick tasks on the go and multitasking -- think about how often you switch between apps in a single iPad session. It's also used (mostly) around the house as a secondary lightweight computer, mainly for consuming entertainment content -- it's much easier to watch video or read books on the couch or in bed with a one-pound touch screen device than it is with a full laptop. Yes, it's possible to use the iPad to get "serious" work done, but the apps used to complete that work will be different from traditional PC productivity apps -- simpler, with fewer functions.

Seriously, try and think of a viable use case for a touch-screen version of Office on the iPad. Are financial wizards going to enter data and run macros by tapping on their screens from the bed? Are marketers going to create PowerPoint presentations on the train? Are execs going to fire up the iPad version of Word during meetings so they can take notes with pretty formatting and change-tracking?

No. If you're a regular Office user, you already have a computer with a keyboard, probably a laptop. If you need to use Office, you're going to use that computer. In the last case, Microsoft already sells a note-taking app that was built for touch screens -- it's called OneNote, and it's been available for iPad since December 2011 (and iPhone before that).

Sure, users might need to take a quick look at an Office-formatted file on their iPad between meetings or during their commute, maybe make a couple changes. Here, Microsoft already has Office Web Apps, but it's pretty limited; users can also download Office Mobile, but it was designed for the iPhone, not the iPad. There are also plenty of third-party tools that suffice for this quick viewing and editing, but their creators may not keep up with the latest changes to Office formats or map to Microsoft's broader business goals -- just look at how Google has slowly brought Quickoffice into the Google fold, killing links to Dropbox and requiring a Google Account to use it, for example.

So for customers who've already paid for an Office 365 subscription, having a touch-friendly version of Office that's designed for the iPad will make these occasional use cases easier.

But for the rest of the world it's a big yawn. Office is a PC product from the PC era. It's big, complicated, almost infinitely backwards-compatible, and full of every possible feature that everybody might need. That's great for the PC, but it's not how mobile computing works.

Moving a full touch-screen version of Office to the iPad solves no real problem for anybody. If there's ever going to be mass adoption of mobile productivity apps, it'll come from apps that were designed for mobile first, like Quip, Box, or Slack.